1. Field of the Invention
A certain aspect of the present invention relates to a fusing device and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction peripheral having functions of those apparatuses generally includes a fusing device for fusing a toner image transferred onto a recording medium (hereafter may be called “paper” but not limited to a sheet of paper). A heat roller fusing device, which is widely used as the fusing device, includes a fusing roller including a heat source such as a halogen heater and a pressure roller pressed against the fusing roller. Paper with an unfused toner image is heated and pressed while being conveyed through a nip formed between the fusing roller and the pressure roller.
There is another type of fusing device called a belt fusing device. A belt fusing device includes a heating roller including a heat source such as a halogen heater, a fusing roller, an endless fusing belt stretched over the heating roller and the fusing roller, and a pressure roller pressed against the fusing roller via the fusing belt. Paper with an unfused toner image is heated and pressed while being conveyed through a nip formed between the fusing belt and the pressure roller. Since a fusing belt has a comparatively low heat capacity, a belt fusing device makes it possible to reduce the warming-up time and to save energy.
The heat roller fusing device and the belt fusing device as described above are called heat fusing devices. In a heat fusing device, the fusing roller or belt that contacts a toner image fused onto paper is coated with fluoroplastic having excellent release characteristics and a separating claw is provided to separate the paper from the fusing roller or belt. The separating claw contacts the fusing roller or belt and therefore tends to scratch the surface of the fusing roller or belt. Such a scratch, for example, results in an undesired stripe in a formed image.
In the case of a monochrome image forming apparatus, the fusing roller is typically made of a Teflon®-coated metal roller that is not easily damaged even when the separating claw touches its surface and has a long service life. Therefore, in such a monochrome image forming apparatus, a separating claw is popularly used to prevent a “paper-winding jam” where paper winds around the fusing roller and gets stuck. Meanwhile, in a color image forming apparatus, to improve the color development, the surface layer of a fusing roller or belt is typically made of fluorine-coated silicone rubber (e.g., a PFA tube with a thickness of several tens μm) or silicone rubber on which oil is applied. However, a surface layer made of such a material is soft and easily scratched. Such a scratch on the surface layer results in an error such as an undesired stripe in a fused image. Therefore, in a color image forming apparatus, a noncontact separating method is preferably used instead of a contact separating method using, for example, a separating claw.
With a noncontact separating method, if the adhesion between toner (toner image) and the fusing roller is strong, paper with a fused toner image easily winds around the fusing roller and causes a paper-winding jam. Particularly, when forming a multicolor image, the adhesion between a toner image and the fusing roller increases because multiple toner layers of different colors are stacked to form the toner image and this in turn increases the chance of the paper-winding jam. Currently, the following noncontact separating methods are mainly used:
(1) Noncontact Separating Plate Method
In this method, a separating plate extending in the length or width direction of the fusing roller or belt is used. The separating plate is disposed in parallel with the fusing roller or belt, and a minute gap (about 0.2-1 mm) is provided between the separating plate and the fusing roller or belt.
(2) Noncontact Separating Claw Method
In this method, multiple separating claws arranged at predetermined intervals are used. A minute gap (about 0.2-1 mm) is provided between the separating claws and the fusing roller or belt.
(3) Self-Stripping Method
In this method, the fusing device is configured such that paper naturally or automatically separates from the fusing roller or belt due to the elasticity of the paper and the curvature of the fusing roller or belt.
However, the above methods have problems as described below. With any one of the methods (1) through (3), a gap is present between the fusing roller or belt and a guide plate guiding paper to the exit of the fusing unit. For example, thin paper, paper with a small top margin, or paper with a solid image such as a photograph may pass through the gap without being separated from the fusing roller or belt and may cause a paper-winding jam. Also, with the methods (1) and (2), paper may get stuck at the separating plate or the separating claws and cause a paper jam.
To solve or reduce the above problems of noncontact separating methods, it is proposed to jet air to a paper-separating position. For example, Japanese Patent Application Publications No. 61-59468, No. 2007-079411, and No. 2007-240920 disclose methods for jetting air to a paper-separating position.
In a configuration disclosed in JP61-59468, a separating claw is provided near a fusing roller and an air outlet for jetting air to the leading edge of paper adhering to the fusing roller is provided at the tip of the separating claw.
JP2007-079411 discloses a configuration including a support part having a hole for allowing air to pass through and jetting parts each having a jetting hole for jetting air to the leading edge of paper. The jetting parts are supported by the support part and can rotate independently from each other. This configuration makes it possible to change air-jetting directions of the jetting parts according to the shape of paper.
In a configuration disclosed in JP2007-240920, compressed air is jetted from an air nozzle to the leading edge of paper that has just passed through a nip of a fusing device to separate the paper from a fusing roller. Also in the disclosed configuration, when paper sheets are consecutively conveyed to fuse toner images onto the paper sheets, compressed air is consecutively jetted onto the outer surface of the fusing roller at positions between the paper sheets being conveyed. This configuration prevents causing temperature differences on the outer surface of the fusing roller and thereby prevents causing uneven glossiness on the paper sheets.
In the configuration of JP61-59468, the air outlet is formed at a position slightly apart from the tip of the separating claw. Still, however, when paper with a small basis weight carries a large amount of unfused toner, the paper may contact the separating claw after going through the nip and being separated from the fusing roller, and the toner adheres to the separating claw. The adhering toner may prevent smooth conveyance of paper, cause an undesired stripe in an image, or block the air outlet. Such adhering toner cannot be easily removed.
In JP2007-079411, a separating part functioning as a paper separating plate and the jetting hole are integrated as the jetting part. With this configuration, the smooth flow of air is prevented if the gap between the jetting part and a fusing roller is reduced. In the configuration disclosed in JP2007-240920, a guide part is disposed to protrude from the air nozzle toward the nip and the paper separated from the fusing roller is guided by the guide part so as not to touch the air nozzle. The air nozzle has a horizontal opening with a length greater than the length of the fusing roller and configured to cool the entire length of the outer surface of the fusing roller. However, since the main object of this configuration is to prevent uneven glossiness on paper sheets being conveyed consecutively, no further mechanism is provided to reliably separate paper from the fusing roller. Particularly in the case of a high-speed color image forming apparatus, the performance of the guide part used as a separating plate in separating paper from the fusing roller improves as the gap between the fusing roller and the guide part decreases. However, if the gap between the fusing roller and the guide part extending in the length direction of the fusing roller is reduced excessively, the flow of air jetted to the entire length of the fusing roller may be blocked and the performance of the air nozzle in separating paper from the fusing roller may be reduced.